Steam generators are designed to produce a large amount of steam to use the moisture of the steam itself. Most of portable steam generator is comprised of water tank wherein electric heaters are installed. Those electric steam generators are easy to make in a compact module and easy to handle. However, due to the inner structure of the steam generating vessel, water tank, there is limit to raise the pressure of the generated steam. The other drawback of those electric heaters is the high electricity cost. The low pressure steam produced by the electric steam generator is not high enough to wash out dirt from a vehicle. Traditional boilers can produce steam of higher pressure and temperature more easily. However, they are designed to utilize the high temperature and pressure of the steam, rather than the moisture of the steam. Those boilers have many tubes in the steam generating vessel to increase the heat transfer capacity. Those boilers are economical for producing steam at a pressure higher than 10 Kg/cm2. The production rate of the steam increases with the number of pipes in the steam generating chamber. Most of all, those boilers are too heavy to manufacture in a compact module.